chondrites would be left in these orbits 
to fall to the earth today. The process 
sketched earlier, wherein the gravi- 
tational tugging of Mars or Jupiter 
gradually warps the orbits of aster- 
oidal fragments until they cross the 
earth’s orbit, is much too slow, taking 
many millions of years. 
One proposal concerning the source 
of ordinary chondrites is that they are 
not derived from the asteroid belt at 
all, but from comets. Comets are small 
masses of ice and grains of interstellar 
dust, in which the “ice” is not only 
the water ice we are familiar with 
but also frozen compounds of ammo- 
nia and other volatile substances. 
When comets approach the sun, their 
ices begin to vaporize and stream out, 
giving rise to the tails and fuzzy cloud- 
like masses, or comas, we associate 
with these objects. Dust particles can 
be seen streaming away from comet 
nuclei, as the ices that enclosed them 
waste away. 
Comets are thought to be icy plan- 
etesimals that accumulate in the cold 
outer reaches of the solar system, ana- 
logues of the rocky planetesimals that 
are preserved in the asteroid belt. 
They were not stored in the early solar 
system, however, but thrown out of 
it by the mighty gravitational forces 
of the outer planets — Jupiter, Saturn, 
Uranus, and Neptune — when they 
strayed too close to one of them. Most 
ejected planetesimals were lost forever 
to interstellar space, but some re- 
mained in slow, coasting orbits around 
the sun, far outside the orbit of Pluto. 
These orbits, too, change because of 
the gravitational tugging on them of 
A real lunar crater called 
Eratosthenes was created by the 
impact of a meteorite. This 
photograph was taken during the 
Apollo 17 mission, the last of the 
manned lunar flights, launched in 
December 1972. 
NASA 
stars in the galaxy other than our sun, 
and every year several icy planetes- 
imals are sent plunging back into the 
solar system. There, additional grav- 
itational interactions with the plan- 
ets can place a steady and fairly co- 
pious flow of them in Earth-crossing 
orbits similar to those of the ordinary 
chondrites. If the nuclei of comets 
contain large rocks and boulders as 
well as dust, they would be a possible 
source of chondritic meteorites. 
A serious difficulty with this hy- 
pothesis is that the character of the 
rock in ordinary chondrites shows it 
has been metamorphosed, or trans- 
formed, by high temperatures 
(roughly 1,000°C) over periods of mil- 
lions of years. This is hard to reconcile 
with a former existence in a mass of 
ice. I have suggested a variation on 
this theme, namely, that rocky plan- 
etesimals from the inner solar system 
were gravitationally ejected just as icy 
planetesimals were from the outer so- 
lar system. Some of these earthy ob- 
jects ended up in the same slow orbits 
as the icy planetesimals, and their 
orbits subsequently were changed by 
the same gravitational interactions 
that govern the comets. Since these 
objects would move very fast when 
they enter the inner solar system and 
do not conspicuously stream gas, they 
would escape the notice of astrono- 
mers. Such objects could be fed into 
Earth-crossing orbits at a rate rapid 
enough to replace losses, and unlike 
the icy planetesimals, it is easy to pic- 
ture them having high-temperature 
histories. 
Whatever their origin, the extrater- 
restrial objects that collide with the 
59 
